Semi-fluorinated p-terphenyl liquid crystals

Article abstract of DOI:10.1016/j.jfluchem.2018.11.011

p-Terphenyls carrying two or four fluorine atoms in the central ring were synthesized via Suzuki-Miyaura cross-coupling reaction using phenylboronic acids and fluorinated dibromobenzenes. By calorimetric and X-ray diffraction analyses it was found that th

Full text of DOI:10.1016/j.jfluchem.2018.11.011

JournalofFluorineChemistry218(2019)42–50
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Semi-fluorinated p-terphenyl liquid crystals
Moisés Alberto Valdés-Pech, Leticia Larios-López, Rosa Julia Rodríguez-González,
Isaura Felix-Serrano, Nayely Trejo-Carbajal, Dámaso Navarro-Rodríguez⁎
Departamento de Materiales Avanzados, Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna 140, C.P. 25294, Saltillo, Coahuila, Mexico
A R T I C L E I N F O
A B S T R A C T
Keywords:
p-Terphenyls carrying two or four fluorine atoms in the central ring were synthesized via Suzuki-Miyaura cross-
coupling reaction using phenylboronic acids and fluorinated dibromobenzenes. By calorimetric and X-ray dif-
fraction analyses it was found that the p-terphenyls carrying two fluorine atoms develop nematic and/or smectic
(A and C) phases. On the contrary, the p-terphenyls substituted with four fluorine atoms are not liquid crystals.
Differences found in the thermal behavior are explained in terms π – πF interactions arising from the electron
withdrawing ability of the fluorine atom. Such π – πF interactions let the p-terphenyls stack in a staggered
configuration that explains the stability of the smectic C phase. On the other hand, the semi-fluorinated p-
terphenyls absorb and emit in the blue region of the electromagnetic spectrum; a measured large Stokes shift
indicates that these compounds are nearly transparent to their own emitted light. This optical characteristic,
combined with liquid crystals properties of low order, makes these new p-terphenyls potential candidates for
linearly polarized blue-emitting layers in electroluminescent devices.
Fluorine
p-terphenyl
Liquid crystal
Fluorescence
1. Introduction
mesophases, induces negative dielectric anisotropies and chirality, etc.
[11–14] p-Terphenyl liquid crystals substituted with cyano groups have
p-Terphenyls substituted with long terminal chains and short lateral
groups (methyl, methoxy, cyano, halogen, etc.) are able to develop li-
quid crystal phases of low order like nematic and smectic-A phases
[1,2]. Under appropriate illumination they are also able to fluoresce in
the blue region of the electromagnetic spectrum [3]. In addition, p-
terphenyls have excellent thermal stability [4], high dielectric aniso-
tropy [5,6], and high capacity to induce birefringence [7]. These and
other properties have been modified (or improved) by exchanging some
of their hydrogens for other atoms or short chemical groups. Fluorine,
the most electronegative element, has undoubtedly been the most ex-
plored substituent in p-terphenyls [8,9]. It turns out that fluorine and
hydrogen atoms are similar in size (1.47 and 1.2 Å, respectively), and
therefore, the one can replace the other without causing major changes
in conformation, although, the electron density distribution and the
intermolecular interactions can be altered, leading to different stacking
and properties [2,10]. It has been generally found that a partial ex-
change of hydrogens by fluorine atoms in p-terphenyl liquid crystals
shifts the mesomorphic behavior towards lower temperatures, induces
the formation of low-ordered (low viscous) mesophases such as the
nematic and smectic C phases, enlarges the temperature interval of
also been the subject of numerous reports, particularly those carrying
this group at one of its two ends [15,16]. Such cyano-substituted p-
terphenyls possess a strong polarization moment due to the high elec-
tron withdrawing character of the cyano group. Furthermore, p-ter-
phenyls carrying cyano groups show fairly good thermal, chemical, and
photochemical stabilities [17]. p-Terphenyls carrying both fluorine and
cyano groups have already been explored [2,16,18], but in only few of
them both groups are in lateral positions. Herein, we report on the li-
quid-crystalline and optical properties of didodecyloxy-p-terphenyls
laterally substituted with fluorine and cyano groups.
2. Experimental
2.1. Materials
All reagents were purchased from Aldrich and used without further
purification unless otherwise noted. Reactive grade acetone, methanol,
chloroform (CHCl3), and hexanes were purchased from J. T. Baker and
were used without further purification. Tetrahydrofuran (THF) was dried
over a sodium/ benzophenone complex and distilled right before use.
⁎ Corresponding author.
E-mail addresses: moises.valdes.psg@ciqa.edu.mx (M.A. Valdés-Pech), leticia.larios@ciqa.edu.mx (L. Larios-López),
julia.rodriguez@ciqa.edu.mx (R.J. Rodríguez-González), isaura.felix.pd@ciqa.edu.mx (I. Felix-Serrano), nayely.trejo.pd@ciqa.edu.mx (N. Trejo-Carbajal),
damaso.navarro@ciqa.edu.mx (D. Navarro-Rodríguez).
https://doi.org/10.1016/j.jfluchem.2018.11.011
Received 7 August 2018; Received in revised form 16 November 2018; Accepted 22 November 2018
Availableonline29November2018
0022-1139/©2018ElsevierB.V.Allrightsreserved.

M.A. Valdés-Pech et al.
JournalofFluorineChemistry218(2019)42–50
Scheme 1. Route for the synthesis of the semi-fluorinated p-terphenyls. i) Acetone, K2CO3, KI and C12H25Br (or C12H24Br2), 65 °C, 48 h, ii) THF, n-BuLi and
triisopropyl borate, −40 °C / HCl 2 N, 25 °C, and iii) THF, Na2CO3 2 M and Pd[P(C6H5)3]4, 65 °C.
2.2. Synthesis
Four partially fluorinated p-terphenyls were synthesized according
1.23–1.54 (m, CH2, 36 H), 1.8 (m, O–CH2–CH2, 4 H), 4.0 (t,
O–CH2–CH2, 4H, J = 6.6 Hz), 6.98 (d, Ar, 4H, J = 8.8 Hz), 7.19 (t, Ar,
2H, J = 8.9 Hz), 7.51 (d, Ar, 4H, J = 8.5 Hz). FT-IR (KBr, cm−1) 2922
and 2853 (ν CH2); 1610, 1528, and 1491 (ν C = C ring);1258 (ν C–F);
1187 (ν C–O–C); 1032 (ν C–O); 888, 838 and 792 (δ C–H). FABHRMS:
to the route outlined in Scheme 1. Two of them carry two or four
fluorine atoms in the central ring and one dodecyloxy chain at each one
of the two peripheral rings. These p-terphenyls were named T2F and
T4F, respectively. The other two p-terphenyls are similar to T2F and
T4F but they carry one extra cyano group in one peripheral ring, and
were named T2F-CN and T4F-CN, respectively. These two later mole-
cules are end-functionalized with one bromine atom because they will
be used in further reactions aiming to obtain methacrylate monomers
and their corresponding side-chain liquid crystal polymers. Inter-
mediates 1-4 were synthesized according to reported procedures. [3,19]
Precursors 5a and 5b, as well as the four semi-fluorinated p-terphenyls,
were synthesized as described below.
Observed (m/z) [M+
]
634.4547 (100%); Estimated [M+
] (m/z)
634.4561. Anal Calcd for C42H60F2O2: C, 79.45; H, 9.53; F, 5.98; O,
5.04. Found: C, 79.98; H, 9.76.
2.2.2. 4-bromo-2,3,5,6-tetrafluoro-4’-dodecyloxybiphenyl (5b)
A procedure similar to that used for the synthesis of compound 5a
was followed. 5.03 g (16.3 mmol) of 1,4-dibromotetrafluoro benzene,
0.3774 g (0.33 mmol) of Pd[P(C6H5)3]4, 42.5 mL of an aqueous Na2CO3
2 M solution, and 2 g (6.53 mmol) of 4-dodecyloxyphenylboronic acid.
Two products were obtained as white crystals: T4F (yield 22.8%) and
5b (yield 40%).
5b: 1H NMR (CDCl3, δ = ppm) 0.88 (t, CH3, 3H, J = 6.9 Hz),
1.22–1.52 (m, CH2, 18 H), 1.8 (m, O–CH2–CH2, 2 H), 4.02 (t,
O–CH2–CH2, 2H, J = 6.5 Hz), 7.0 (d, Ar, 2H, J = 8.8 Hz), 7.38 (d, Ar,
2H, J = 8.8 Hz).
2.2.1. 4-bromo-2,5-difluoro-4’-dodecyloxybiphenyl (5a)
In a 250 mL three-neck round-bottom flask, under magnetic stirring
and argon atmosphere, 200 mL of THF (freshly distilled), 4.44 g
(16.3 mmol) of 1,4-dibromo-2,5-difluorobenzene, and 0.226 g
(0.2 mmol) of Pd[P(C6H5)3]4, were introduced. This mixture was he-
ated to 65 °C before adding 42.5 mL of an aqueous solution of Na2CO3
2 M. The reaction was held under stirring for 30 min and then 2 g
(6.53 mmol) of 4-dodecyloxyphenyl boronic acid (dissolved in THF)
were added dropwise. 72 h later the reaction was stopped and the
solvent was evaporated. Then, 200 mL of chloroform were added and
the solution was washed three times with water. The chloroform was
evaporated and the remaining solid was dried before being re-dissolved
in hot chloroform. On cooling this solution, a white solid was pre-
cipitated (T2F, yield 28%) and recuperated by filtration. The liquid
phase was evaporated and the recuperated solid was recrystallized from
hot methanol and filtered. 5a was obtained as a white powder (yield
40%).
T4F: 1H NMR (CDCl3, δ = ppm) 0.89 (t, CH3, 6H, J = 6.5 Hz),
1.19–1.55 (m, CH2, 36 H), 1.80 (m, O–CH2–CH2, 4 H), 4.02 (t,
O–CH2–CH2, 4H, J = 6.6 Hz), 7.01 (d, Ar, 4H, J = 8.8 Hz), 7.42 (d, Ar,
4H, J = 8.5 Hz). FT-IR (KBr, cm−1): 2923 and 2854 (ν CH2); 1613,
1530, and 1469 (ν C = C ring); 1250 (ν C–F); 1176 (ν C–O–C); 1028 (ν
C–O); 822 and 725 (δ C–H).
2.2.3. 4-(ω-bromododecyloxy)-2′,5′-difluoro-4″-dodecyloxy-1,1′:4′,1″-
terphenyl-3-carbonitrile (T2F-CN)
In a 250 mL three-neck round-bottom flask, under stirring and argon
atmosphere, 200 mL of THF (freshly distilled), 1.137 g (2.51 mmol) of
5a, and 0.145 g (0.125 mmol) of Pd[P(C6H5)3]4, were introduced. This
mixture was heated to 70 °C before adding 16.3 mL of Na2CO3 2 M
(aqueous solution), and 1.2342 g (3.01 mmol) of 3-cyano-4-(ω-bromo-
dodecyloxy) phenylboronic acid. The reaction was held at 70 °C and
stirring for 72 h. Once this time has elapsed, the solution was allowed to
cool down to room temperature and 100 mL of ethyl ether were added.
The solution was filtered and the solid was transferred to a separation
funnel where it was washed three times with distilled water. The ether
5a: 1H NMR (CDCl3, δ = ppm) 0.88 (t, CH3, 3H, J = 6.6 Hz),
1.19–1.65 (m, CH2, 18 H), 1.8 (m, O–CH2–CH2, 2 H), 4.0 (t,
O–CH2–CH2, 2H J = 6.6 Hz), 6.95 (d, Ar, 2H, J = 8.5 Hz), 7.18 (dd, Ar,
1H, J = 8.9, 6.7 Hz), 7.34 (dd, Ar, 1H, J = 9.5, 5.9 Hz), 7.45 (d, Ar, 2H,
J = 8.5 Hz).
T2F: 1H NMR (CDCl3, δ = ppm) 0.89 (t, CH3, 6H, J = 6.6 Hz),
43

M.A. Valdés-Pech et al.
JournalofFluorineChemistry218(2019)42–50
phase was evaporated and the solid was suspended in CHCl3. Finally,
the product was purified by column chromatography (silica gel) using a
hexanes:ethyl acetate (95:5) mixture as eluent. T2F-CN was obtained as
a white solid (yield 66.7%). 1H NMR (CDCl3, δ = ppm) 0.86 (t, CH3,
3H, J = 6.6 Hz), 1.2–1.6 (m, CH2, 34 H), 1.7–1.9 (m, O–CH2–CH2,
Br–CH2–CH2, 6 H), 3.41 (t, CH2–Br, 2H, J = 6.9 Hz), 4.0 (t, O-CH2–CH2,
2H, J = 6.6 Hz), 4.12 (t, O-CH2–CH2, 2H, J = 6.5 Hz), 6.98 (d, Ar, 2H,
J = 9.1 Hz), 7.05 (d, Ar, 1H, J = 9.1 Hz), 7.15 (dd, Ar, 1H, J = 10.7,
6.6 Hz), 7.22 (dd, Ar, 1H, J = 11.0, 6.6 Hz), 7.5 (dd, Ar, 2H, J = 8.8,
1.4 Hz), 7.7 (dd, Ar, 1H, J = 8.8, 1.1 Hz), and 7.8 (d, Ar, 1H,
J = 1.2 Hz). FT-IR (KBr cm−1): 2919 and 2850 (ν CH2); 2231 (ν CN);
1613, 1527, and 1489 (ν C = C ring); 1269 (ν C–F); 1165 (ν C–O–C);
1025 (ν C–O); 882, 825 and 785 (δ C–H); and 666 (ν C–Br). FABHRMS:
temperatures on cooling from the isotropic liquid. UV–vis spectra
(190–820 nm) were recorded in a spectrophotometer from Shimadzu
(UV-2401PC) using a standard quartz cell and spectrophotometric
grade chloroform at a concentration of around 0.05 mg mL−1. Finally,
the fluorescence in solution (spectrophotometric grade CHCl3 as sol-
vent) was measured in a spectrofluorimeter from Perkin Elmer LS-50B.
Procedures were identical for all samples.
3. Results and discussion
3.1. Synthesis
The semi-fluorinated p-terphenyls were synthesized through three
reactions steps: i) alkylation of the 4-hydroxy-1-bromobenzene (with
and without a cyano group) with a mono or dibromo-terminated do-
decyl chain, ii) formation of the aryl boronic acids (2 and 4 in Scheme
1) using the alkylated bromobenzenes, BuLi and triisopropylborate, and
iii) coupling of the alkylated phenyl boronic acids with a di or tetra-
fluorinated dibromobenzene via the Suzuki-Miyaura cross-coupling
reaction using Pd[P(C6H5)3]4 as catalyst. [20] Here, only the step iii was
detailed in the experimental section because the steps i and ii are similar
to those described in a previous report. [19] As noted in the experi-
mental part, T2F and T4F were obtained as byproducts in the synthesis
of 5a and 5b, respectively. Despite the low yield (28% for T2F and
22.8% for T4F), the obtained amounts were enough for a full chemical,
thermal and optical characterization.
Observed (m/z) [M+] 737.3624 (95.05%); Estimated (m/z) [M+
]
737.3619. Anal Calcd for C43H58BrF2NO2: C, 69.90; H, 7.91; Br, 10.81;
F, 5.14; N, 1.90; O, 4.33. Found: C, 71.24; H, 8.59; N, 1.82.
2.2.4. 4-(ω-bromododecyloxy)-2′,3′,5′,6′-tetrafluoro-4″-dodecyloxy-
1,1´:4′,1″terphenyl-3-carbonitrile (T4F-CN)
A procedure similar to that used for the synthesis of T2F-CN was
followed. 1.06 g (2.17 mmol) of 5b, 0.1181 g (0.1 mmol) Pd[P
(C6H5)3]4, 1 g (2.44 mmol) of 3-cyano-4-(ω-bromododecyloxy) phe-
nylboronic acid, and 13.28 mL of Na2CO3 2 M (aqueous solution). The
product was purified by using a silica gel chromatographic column and
a hexanes:ethyl acetate (9:1) mixture. T4F-CN was obtained as a white
solid (yield 81%). 1H NMR (CDCl3, δ = ppm) 0.8 (t, CH3, 3H,
J = 6.6 Hz), 1.2–1.6 (m, CH2, 34 H), 1.7–1.9 (m, O-CH2-CH2, Br-CH2-
CH2, 6 H), 3.4 (t, CH2-Br, 2H, J = 6.9 Hz), 4.01 (t, O-CH2-CH2, 2H,
J = 6.5 Hz), 4.12 (t, O-CH2-CH2, 2H, J = 6.5 Hz), 7.02 (d, Ar, 2H,
J = 8.8 Hz), 7.09 (d, Ar, 1H, J = 8.8 Hz), 7.45 (d, Ar, 2H, J = 8.5 Hz),
7.66 (d, Ar, 1H, J = 8.8 Hz), and 7.73 (s, Ar, 1H,). FT-IR (KBr cm−1):
2919 and 2851 (ν CH2); 2232 (ν CN); 1612, 1525, and 1469 (ν C = C
ring); 1256 (ν C–F); 1181 (ν C–O–C); 1027 (ν C–O); 822 and 724 (δ
C–H); and 636 (ν C–Br). FABHRMS: Observed (m/z) [M+] 773.3429
The structure and purity of the four synthesized p-terphenyls were
confirmed by 1H NMR spectroscopy. Since all spectra are similar, only
those corresponding to T2F-CN and T4F-CN were selected as examples
(Fig. 1). It can be noted that, except for the typical singlet of CDCl3
(7.26 ppm), all signals correspond to protons of the synthesized mole-
cules (see inserts in Fig. 1). The only difference between the two spectra
of Fig. 1 is the complex signal centered at 7.18 ppm in the spectrum of
T2F-CN, and that corresponds to the d and e protons. Such complex
signal is a double of doublets that indicates that protons d and e are
coupled with the neighboring fluorine atoms. 13C NMR and 2D NMR
experiments (Supplemental online information, S1-S6) supported the ac-
curate assignment.
(20.93%); Estimated (m/z) [M+
]
773.3431. Anal Calcd for
C
43H56BrF4NO2: C, 66.66; H, 7.29; Br, 10.31; F, 9.81; N, 1.81; O, 4.13.
Found: C, 67.09; H, 7.51; N, 1.83.
2.3. Instruments
3.2. Thermal behavior
The chemical structure of intermediates and final products was
confirmed by proton and carbon nuclear magnetic resonance (1H NMR
and 13C NMR) spectroscopy using a Jeol 300 MHz spectrometer and
CDCl3 or methanol-d4 as solvent. The chemical structure was also stu-
died by Fourier Transform Infrared (FTIR) spectroscopy (ATR method)
using a 550 Nicolet Magna spectrophotometer. The elemental analysis
was performed with a PerkinElmer 2400 Series II CHNS/O Elemental
Analyzer. High resolution mass spectrometry was recorded on a Jeol
JMS-700 MStation, Ion Mode: FAB+ (Fast atom bombardment). The
thermal stability of vacuum dried samples was determined with a
thermogravimetric analyzer (TGA) from DuPont Instruments (TGA 951)
The four synthesized p-terphenyls were first analyzed by TGA
(Supplemental online information, S7) to determine their initial decom-
position temperature, which was later considered as a limit temperature
for their thermotropic analysis by DSC, POM and XRD. All four p-ter-
phenyls resulted thermally stable up to 300 °C as is typical for p-ter-
phenyl derivatives. [4] This temperature is far higher than the clearing
temperature of any of them.
The DSC thermograms (Fig. 2) of two p-terphenyls showed multiple
transitions associated to a mesomorphic behavior. One corresponds to
T2F, which, upon cooling, developed four thermal transitions around
121, 119, 117 and 108 °C, and the other one corresponds to T2F-CN,
which displayed two thermal transitions near 100 and 64 °C. As noted
in the DSC traces, both T2F and T2F-CN are enantiotropic liquid crys-
tals. T4F showed only one thermal transition around 131 °C, whereas
T4F-CN displayed three broad thermal transitions corresponding to li-
quid-solid (108 °C), solid-solid (98 °C), and solid-solid (57 °C) transi-
tions.
connected to a N2 vector gas, and heating at constant rate (10 °C min–1
)
from 30 to 700 °C. The thermal behavior (phase transitions) was de-
termined in a FP94HT differential scanning calorimeter (DSC) from
Mettler at heating and cooling rates of 3 °C/min; reported traces cor-
respond to the first cooling and second heating scans. The optical tex-
tures of mesophases were registered at different temperatures on
cooling from the isotropic liquid, using a polarizing optical microscope
(POM) from Olympus, coupled to a FP82HT heating plate from Mettler.
The X-ray diffraction (XRD) analysis was performed in a SWAXS from
Anton Paar (SAXSess mc2) equipped with a sample holder unit (TCS
300-C), an image plate detector, and a temperature control unit
(TCU50). X-rays (Cu kα radiation; λmax = 0.1542 nm) were generated
at 40 kV and 50 mA. Each sample (finely powdered) was introduced
into a glass capillary with outer diameter and wall thickness of 1.0 and
0.01 mm, respectively. XRD patterns were captured at different
A non-fluorinated p-terphenyl (homologous to T2F and T4F) car-
rying one dodecyloxy chain at each one of the peripheral rings was
synthesized and thermally characterized by us in a previous work [21].
Its DSC thermogram displayed four mesomorphic regions that were
associated to different smectic phases as therein deduced from XRD
results. The melting and clearing temperatures of such a non-fluori-
nated p-terphenyl are 130 and 205 °C, respectively. These transition
temperatures are higher than the corresponding ones for T2F (113 and
44

M.A. Valdés-Pech et al.
JournalofFluorineChemistry218(2019)42–50
Fig. 1. 1H NMR spectra of T2F-CN and T4F-CN (CDCl3). In order to make comparison between spectra easier, protons were signaled with the same letters (note that
letters d and e were omitted in the spectrum of T4F-CN).
122 °C, respectively), although the melting temperature (130 °C) is
close to that shown by T4F (∼134 °C). From this comparison it is clear
that the substitution of two hydrogens for two fluorine atoms has
produced a shifting of the mesomorphic behavior towards lower tem-
peratures. The substitution of the all four hydrogens of the central ring
(T4F) resulted in a p-terphenyl derivative showing no liquid crystalline
properties.
[9]. For p-terphenyls substituted with two fluorine atoms in the central
ring (in ortho position the one to the other) they observed only one
nematic phase, while for those having the two fluorine atoms in a
peripheral ring they have encountered nematic and smectic A (or
smectic C) phases. Goodby et al. have found similar results in p-ter-
phenyls substituted with two fluorine atoms either in the central or in a
peripheral ring [2]. Our T2F molecule (fluorine atoms in para position)
showed these three mesophases with the smectic C phase over a larger
temperature interval. T2F-CN developed only one mesophase, although
in this compound the thermal behavior depends on the effects of both
fluorine and cyano groups.
The phase transitions detected by DSC were next corroborated by
polarizing optical microscopy. Observations were made at different
temperatures on cooling from the isotropic phase. At 121 °C, T2F
showed droplets (Fig. 3A) that merged into a schlieren texture (Fig. 3B)
typical of a nematic phase. Then, at 119 °C the schlieren texture
changed into a focal conic fan texture (Fig. 3C) that almost instantly (at
118 °C) turned into a broken fan-shape texture (Fig. 3D). [22] The
mesophase observed between 119 and 118 °C was not detected in fur-
ther XRD studies, but according to the optical texture it can be attrib-
uted to a smectic-A phase. On the other hand, T2F-CN displayed at
99 °C a focal-conic fan texture (micrograph not shown here) that remain
unchanged up to 65 °C where it crystallized.
The nature and structure of the observed mesophases was assessed
from the X-ray diffraction patterns recorded at different temperatures
on cooling from the isotropic phase. At 120 °C, T2F displayed one broad
peak at both low and wide angles of the X-ray pattern (Fig. 4), corro-
borating the presence of a non-ordered phase of the nematic type. On
cooling to 116 °C, the diffuse peak at low angles turned into a sharp
Bragg reflection (indexed as 001) while the broad peak remained un-
changed. The sharp peak was attributed to the stacking period (d001) of
a smectic phase. d001 (41 Å) was then compared with the length of the
molecule in its most extended conformation L (45.4 Å), calculated by
molecular modelling software from Spartan (Spartan 10). Results
showed that d001/L is slightly smaller than unity, suggesting a single
layer stacking with molecules tilted (25.4°) with respect to the normal
of the smectic plane [23]. On the other hand, T2F-CN showed a sharp
Bragg reflection (001) at low angles and a broad peak at wide angles,
also indicating a smectic phase of low order. In comparing d001 (39.6 Å)
In most works on the thermotropic behavior of fluorine-substituted
p-terphenyl liquid crystals, nematic and/or low ordered smectic phases
were reported. For instance, Choluj et al., studied the thermotropic
behavior of series of p-terphenyls substituted with one, two or three
fluorine atoms, and they found that the number of mesophases de-
creases as the number of fluorine atoms in the p-terphenyl core in-
creases, in such a way that those p-terphenyls substituted with three
fluorine atoms (in a variety of positions) develop only a nematic phase
45

M.A. Valdés-Pech et al.
JournalofFluorineChemistry218(2019)42–50
Fig. 2. DSC thermograms of the semi-fluorinated p-terphenyls.
Fig. 3. Optical textures of T2F captured at different temperatures (TA = 121 °C, TB = 119 °C, TC = 118.5 °C, and TD = 116 °C) on cooling from the isotropic phase.
46

M.A. Valdés-Pech et al.
JournalofFluorineChemistry218(2019)42–50
Fig. 4. X-ray diffraction analysis of p-terphenyls substituted with two fluorine atoms registered at various temperatures upon cooling from the isotropic liquid.
Table 1
Thermal properties and structural parameters for T2F and T2F-CN.
b)
Molecule
T
d001
L
d/L
Stacking
Tilt angle
(°)
Thermal transitions
(°C)
(°C)
(Å)
(Å)
a)
T2F
116
80
41
45.4
46.0
0.9
Single layer
Single layer
25.4
I 121 N 119 SmA 118 SmC 108 Cr
I 100 SmC 64 Cr
T2F-CN
39.6
0.86
30.6
a)For the SmC phase. b)Upon cooling.
Fig. 5. X-ray diffraction patterns of p-terphenyls substituted with four fluorine atoms registered at two different temperatures.
with L (46.0 Å) a single-layered smectic C phase was also concluded for
this derivative. The thermal transitions and the structural parameters
for T2F and T2F-CN are gathered in Table 1. For T4F and T4F-CN the
multiple peaks across the X-ray pattern confirmed their crystalline
nature (Fig. 5).
the electron density distribution (Fig. 7), the π – πF interaction must be
stronger in the stacks of T4F than in those of T2F. The measured higher
melting temperature of T4F is possibly due to a close packing of mole-
cules arising from such stronger π – πF interaction. For T2F, the electron
density distribution lets the molecules stack through π – πF interactions
but much weaker. Its low melting temperature, as well as its ability to
develop liquid crystals phases, are likely associated to these weaker in-
teractions. The presence of a cyano group in p-terphenyls (T2F-CN and
T4F-CN) also affects the way they arrange by themselves in a mesophase
or in a crystalline structure. The relatively high bulkiness of this group
(as compared to hydrogen and fluorine atoms) hinders the close packing
of molecules, although, its high electron withdrawing character produces
other type of interactions, like the C–H… N interaction that is relatively
weak but that can determine the crystal packing of molecules carrying
both fluorine atoms and cyano groups. This is the case of some fluor-
obenzonitriles whose crystal packing is directed by the C–H…N inter-
action [27]. In our T2F-CN and T4F-CN molecules, the C–H…N inter-
action can contribute to determining both the molecular packing and the
thermal properties. Compared to T2F, T2F-CN showed lower melting and
clearing temperatures, suggesting that the cyano group perturbs the in-
teraction between p-terphenyls but to a limited extent since it is still able
to develop liquid crystal properties. T4F-CN showed a lower melting
temperature than T4F, but even so, it is not able to develop a liquid
crystal behavior.
Partially fluorinated p-terphenyl liquid crystals have intensely been
studied from the stand point of thermal properties and structural char-
acteristics [2,16]. In general, the fluorine atoms tend to depress the
melting temperature, decrease the mesomorphic stability, and increase
the probability of mesophases of low order like nematic, smectic A and
smectic C phases [9,10]. A full exchange of hydrogens by fluorine atoms
in the benzene ring inverses its electron density distribution [24]. In
polyaromatic molecules having both perfluorinated (electron poor) and
non-fluorinated (electron rich) rings, the π – πF interactions are supposed
to prevail over the π – π interactions [25]. The π – πF interactions may
therefore stabilize the stacking of molecules in a specific configuration,
depending on which rings (and how) are fluorinated [26]. For p-ter-
phenyls carrying fluorine atoms in the central ring, zig-zag (Fig. 6B) and
staggered (Fig. 6C) configurations are expected to occur since both
configurations let the electron rich rings associate with the electron poor
rings of neighboring molecules through π – πF interactions. The stag-
gered packing has already been proposed by Goodby et al. for fluorine-
substituted p-terphenyls [2]. Therein, it was suggested that the staggered
arrangement induces the formation of tilted smectic phases, as was in-
deed found in the p-terphenyls studied the present work. According to
47

M.A. Valdés-Pech et al.
JournalofFluorineChemistry218(2019)42–50
Fig. 6. Stacking of T2F-CN in the
smectic phase. The zig-zag (B) and
staggered (C) configurations are stabi-
lized by
π
–
πF interactions.
Configuration A is not favorable from
the interactional point of view. The
alkoxy chains were omitted.
Their relatively large band gap energy (∼ 3.5 eV) makes them blue-
emitting materials [32]. The introduction of high electron-withdrawing
substituents, like fluorine and cyano groups, lowers the HOMO/LUMO
levels and therefore facilitates or allows the tuning of some electro-
optical processes, like the electron injection in OLEDs [33].
The UV–vis absorption and emission spectra of the semi-fluorinated
p-terphenyls were obtained in solution, using spectrophotometric grade
chloroform as solvent. The absorption spectra of T2F and T2F-CN dis-
played two partially superposed bands with maxima (λmax) around 280
and 303 nm (Fig. 8), whereas those of T4F and T4F-CN showed one
single band with λmax around 284 nm (Fig. 9). As known, the shape,
intensity, and position of the absorption bands of π-conjugated mole-
cules depend on the conjugation length, conformation (planarity),
nature and number of substituents, among others [3,16,34]. In solution,
the band position also depends on the solvent polarity [35]. For the p-
terphenyl without fluorine atoms (control) the measured λmax in
chloroform is 268 nm [3], therefore, the substitution of two or four
hydrogens for fluorine atoms has produced a bathochromic effect (red
shift). T4F and T4F-CN absorb at lower wavelength (λmax = 284 nm) as
compared to T2F and T2F-CN (λmax = 303 nm) due to steric effects that
make the tetrafluoro-substituted p-terphenyls more twisted than the
difluoro-substituted ones, perturbing in larger extent the conjugation of
π-orbitals (associated to the planarity). A similar effect was observed by
Seed et al. in studying ortho and meta monofluoro-substituted biphenyls
[34]. These authors found that the meta-substituted biphenyls absorb at
higher wavelength (lower energy) than the ortho-substituted ones due
to their lower interannular torsion angle that makes the π-conjugation
more effective. The two absorption bands in the spectra of T2F and T2F-
CN were attributed to transitions between localized orbitals (lower
wavelengths) and conjugation of phenyl rings (high wavelengths) [3].
The molar absorptivity (ε) of each semi-fluorinated p-terphenyl was
graphically determined (Fig. 10), and resulting values are gathered in
Table 2. It can be readily noticed that ε increases markedly from the
two to the four fluorine-substituted p-terphenyls, meaning that the in-
troduction of more fluorine atoms makes the p-terphenyl core a
stronger absorbing specie. As noted, neither the shape nor the position
of absorption bands was affected by the cyano group. A similar result
was observed in a previous study on the UV–vis absorption character-
istics of p-terphenyls (non-fluorinated) laterally substituted with zero,
one or two cyano groups [32]. The optical band gap (Eg opt) and the
fluorescence quantum yield (Φ) were measured in solution (CHCl3 as
solvent) for the four semi-fluorinated terphenyls (Table 2). Egopt values
of 3.54 and 3.8 eV indicate that all four compounds are semiconducting
materials. The quantum yield (between 44 and 60%) is relatively high
and comparable to that reported for other terphenyls [32,36].
Fig. 7. Electron density distribution of T2F and T4F stabilized through π – πF
interactions. Modeled molecules have two methoxy groups (one at each end of
the p-terphenyl) instead of two dodecyloxy chains.
Fig. 8. UV–vis absorption and emission spectra of p-terphenyls substituted with
two fluorine atom and with (T2F-CN) or without (T2F) cyano group. CHCl3 was
used as solvent.
All four fluorinated compounds were next excited with a mono-
chromatic UV light 10 nm below λmax. The recorded spectra showed
that all four chromophores fluoresce at almost the same wavelength
interval with maximum around 367 nm (Figs. 8 and 9). Only T4F
showed a slightly red-shifted (λmax = 372) and broader (HHBW =
90 nm) emission band, suggesting that for this compound more con-
formers in the excited state are emitting. The large Stokes shift (△ν ∼
8200 cm−1), associated to a change from a high energy twisted con-
former in the ground state to a lower energy planar (quinoid) con-
former in the excited state, makes these materials nearly transparent to
their own emitted light [32,35]. These preliminary results may be
Fig. 9. UV–vis absorption and emission spectra of p-terphenyls substituted with
four fluorine atoms, and with (T4F-CN) or without (T4F) cyano group. CHCl3
was used as solvent.
3.3. Optical properties
Besides their ability to develop liquid crystal properties, p-terphe-
nyls are known for their optical and photo-physical properties that have
resulted potentially useful for applications as light emitting materials
[28], wavelength shifters [29], scintillators [30], among others [31].
48

M.A. Valdés-Pech et al.
JournalofFluorineChemistry218(2019)42–50
Fig. 10. UV–vis absorption spectra of T2F at different concentrations (left), and absorbance (measured at two λmax) versus concentration plots for T2F and T2F-CN
(right). CHCl3 was used as solvent.
Table 2
UV–vis absorption and emission properties of the semi-fluorinated p-terphenyls in solution (chloroform as solvent).
λmax
(nm) abs
HHBWabs
λmax
(nm) emi
HHBWemi
△υ (cm−1
)
ε
Egopt
Φ
(%)
(nm)
(nm)
(M−1 cm−1
)
(eV)
T2F
280
302
285
280
304
283
66
367
47
5,864
33,821
31,265
38,249
25,119
20,410
40,595
3.54
52
T4F
45
66
372
367
90
47
8,206
5,646
3.80
3.54
60
56
T2F-CN
T4F-CN
51
368
65
8,161
3.80
44
complemented with further work on photo-physical properties to
evaluate the potential application as for instance in linearly polarized
organic light-emitting diodes (OLEDs) profiting of the liquid-crystalline
properties and the high energy emission (blue region) shown by these
semi-fluorinated p-terphenyls.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jfluchem.2018.11.011.
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